Allow shortening reborrows

Generating a call to `as_mut()` let to more restrictive borrows than
what reborrowing usually gives us. Instead, we change the desugaring to
reborrow the pin internals directly which makes things more expressive.

compiler/rustc_span/src/symbol.rs

@@ -1418,7 +1418,6 @@ symbols! {
         pic,
         pie,
         pin,
-        pin_as_mut,
         pin_ergonomics,
         platform_intrinsics,
         plugin,